(CNN) -- Republican presidential candidate Rudy Giuliani received a "clean bill of health" from doctors Thursday after he was admitted to a St. Louis hospital overnight for flu-like symptoms, said his communications director, Katie Levinson.

Rudy Giuliani's plane returned to the airport when his condition worsened after takeoff.

"Doctors performed a series of precautionary tests, and the results of all the tests were normal," she said in a statement. "The mayor is heading back to New York this afternoon, and he continues to be in high spirits."

Giuliani -- who had been campaigning in Missouri on Wednesday for the February 5 Republican primary -- became ill and then felt worse after his plane took off to return to New York.

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"I would want to make sure there was a chance it would work," Giuliani said from Columbia, Missouri. "I would want to make sure there would be an opportunity to verify whatever it is we were going to do."

Iran, which the United States considers to be a state sponsor of terrorism, has been under pressure from the U.S. and the international community to suspend its development of nuclear technology.

Giuliani, the former mayor of New York, suggested he would meet Iranian President Mahmoud Ahmadinejad, but only if preconditions were established, noting that President Ronald Reagan met with America's adversaries.

During the interview, Giuliani also said waterboarding should not be used in interrogations, but he opened the door to using it in a "once-in-a-lifetime, once-in-a-decade situation."